
Italicized pets are possibly intended to be that small.
Eastern Kingdoms:
Arathi Highlands - Highland Thrashers
Blasted Lands - Grunter
Dun Morogh - Ragged Young Wolves
Elwynn Forest - Blackrock Battle Worgs, Young Wolves
Hillsbrad Foothills - Wild Horses (Pinto color only)
Redridge Mountains - Redridge Foxes, Redridge Fox Kits
Tirisfal Glades - Greater Duskbats
Western Plaguelands - Venom Mist Lurkers
Kalimdor:
Darkshore - some Vile Sprays
Durotar - Lo'Shall, Pygmy Surf Crawler, Surf Crawler
Felwood - Ironwood Buzzers
Maraudon - Deep Borers, Stolid Snapjaws
Ragefire Chasm - Earthborers
Tanaris - Duneclaw Broodlings, Duneshore Crabs
Uldum - Amethyst Scarabs, Emerald Scarabs, Sapphire Scarabs, Turquoise Scarabs
Wailing Caverns - Deviate Adders
Outland:
Hellfire Peninsula - Z'kral (non-pet)
Northrend:
Borean Tundra - Sand Turtles
Grizzly Hills - Bloodmoon Worgen
Sholazar Basin - Hardknuckle Foragers
Cataclysm:
Deepholm - Stone Bats
Tol Barad Peninsula - Baradin Foxes
Vashj'ir - Sandskin Pincers
Oversized: Alpine Hares in Dun Morogh seem to have the opposite problem; they're huge! All of the dragonhawk minipets grew as well. Vicious Black Bears in Hillsbrad Foothills seem overly large, but that might be intentional.